Hepatitis virus infections of humans are being investigated with the objectives of propagating the causative agents in tissue cultures and of studying the interplay between the infecting viruses and naturally infected humans. The significance of host responses is being studied, including investigations of the mechanism of action of the interferons which stemmed from studies originally done with leucocytes and serum from patients with hepatitis B. These investigations are aimed at elucidating interferon species specificity and the kinetics of interferon response.